The present invention is in the field of injection moulding and is more specifically directed to a new and improved double-nozzle injection system for injecting hot molten plastic into a mould.
Injection moulding has been in widespread use for a substantial number of years and has provided an efficient and economical way for producing many items of both simple and complex nature. Such systems employ an extruder capable of providing molten plastic at high pressure into a nozzle block on which a nozzle is mounted for engagement with an injection opening on the mould so that plastic is forced into the injection opening and into the confines of the mould to form the desired item. In moulding operations where the mould has only a single injection opening engageable with a single nozzle, problems of injection are minimal; however, more complex items require moulds having a pair of openings and a pair of nozzles for injecting liquid plastic into the cavities within the mould and it has been found that such systems frequently do not provide satisfactory performance. One of the problems in the use of plural nozzle systems is that the different cavities of the mould being filled by the different nozzles do not have the same volume and flow from one of the nozzles is terminated prior to the flow from the other nozzle. Consequently, the liquid plastic in the runners or conduits in the manifold block which lead directly to the first nozzle becomes stagnant and does not move while the remaining nozzle continues to discharge liquid plastic until its portion of the mould is filled. Stagnation of the plastic in the runners frequently results in overheating of certain areas by the electric heaters provided in the nozzle block so that the plastic becomes burned and discolored or deteriorates to such an extent as to preclude its satisfactory use in the formation of a subsequent item to be moulded. Although the flow rate through the different nozzles can be adjusted to permit completion of the injection from both nozzles at approximately the same time, the foregoing problem of thermal degradation can still occur due to the slow movement of the plastic in the runners supplying a nozzle having a low flow rate. The burning and thermal degradation in most cases is particularly acute in areas such as 90.degree. turns and the like where the liquid plastic can tend to accumulate and remain stationary so as to permit eventual overheating.
Therefore, it is the primary object of this invention to provide a new and improved double-nozzle injector system.
A further object of the present invention is the provision of a new and improved double-nozzle injector system which will avoid thermal damage to molten plastic material being used in the system.